10 gigabit (10GBase-T) Ethernet is the most recent generation of Ethernet network after 10Base-T, 100Base-T and 1 gigabit (1GBase-T) Ethernet. 10GBase-T allows data to be transferred at speeds of 10,000 Mbps or 10 Gbps. 10GBase-T is designed to run over CAT 5 or CAT 5E twisted pair cable which is widely available. CAT 5 cables have four twisted wire pairs of which only two are used for 10Base-T or 100Base-T. In order to achieve higher transmission speeds, 1GBase-T and 10GBase-T Ethernet networks use all four pairs of twisted wire to transmit data.
Power over Ethernet (PoE) is a system in which, in addition to data, electrical power is passed over Ethernet cabling. This enables the elimination of a separate cable solely for power use in favor of a single cable (e.g., via a CAT 5/5E cable) to provide both a data connection and electrical power to devices such as wireless access points, resulting in substantial savings in installation costs. The need for AC power and DC transformers at the powered device location is eliminated.
There are two different ways of feeding power to Ethernet networks, namely spare-pair power and phantom power. In phantom power feed the voltage is coupled to the wire pairs that carry the data signals. Since twisted-pair Ethernet uses differential signaling, the power can be supplied on the same wire pair that carries data. In spare-pair power feed, the voltage is coupled to the unused or spare wire pairs, i.e., the wire pairs that do not carry data. Spare-pair power feed does not apply to 1GBase-T or 10GBase-T Ethernet networks because all eight wire pairs are used for signal transmission, i.e., no spare-pairs are available. Thus, phantom power feed is used in 1 gigabit and 10 gigabit PoE networks. Power is transmitted on the data conductors by applying a common-mode voltage to each pair, either from an endspan power supply within a PoE-enabled networking device (PSE), such as an Ethernet switch, or with a midspan power supply. Endspans are essentially Ethernet switches with PoE circuits added and are typically used in new installations. Midspans are not integrated into the switch but are positioned between it and the powered device (PD). They are usually used when a PD, such as an IP phone or wireless AP, is added to an existing non-PoE network.
Power over Ethernet standards are defined under IEEE 802.3af (2003) and IEEE 802.3at (2009). The original IEEE 802.3af standard specifies a supply of 48 VC and up to 350 mA so that the power source element does not exceed 15.4 watts. After power dissipation in the cable, the power actually delivered to the PD is limited to 12.95 W.
Increasing power delivered to the PD is the main objective of subsequently developed PoE standards. IEEE 802.3at (also known as PoE+, 2009) provides up to 30 W of 50-V DC power at the PSE. After cable loss, 25.5 W is delivered to the PD. IEEE 802.3at added 1GBase-T which uses all four pairs for data transmission. 10GBase-T PoE is not specified in either 1EEE 802.3af or 802.3at.
The relatively small amount of power that can be delivered to PDs under the 802.3at standard (25.5 W), has limited the application of PoE much beyond wireless APs and IP phones. However, the anticipated introduction of a new PoE standard, IEEE 802.3bt, will at least double the power delivered to the PD by allowing power over all four pairs in a standard Ethernet cable. The new standard is also expected to standardize PoE with 10GBase-T Ethernet. The ability to deliver higher power to PDs will expand the applicability of PoE to high-volume applications such as building management applications (many around 50 W), point-of-sale systems (30 to 60 W), and industrial motor control systems (>30 W).
Problems arise in connection with supplying greater amounts of power to 10GBase-T Ethernet networks. Power is typically inserted using center-tapped transformers or autotransformers connected between two pins of each wire pair. The transformers typically include a magnetic core comprising a toroid core with a magnetic winding. As data speeds and power increase in PoE systems, higher magnetizing and DC current bias capabilities are required for the toroidal magnetic cores. Transformer core saturation can limit the current that can be sent to a PD. Transmission characteristics are now specific to bandwidths of 500 MHz. Loading of conventional toroidal transformers or autotransformers on the signal path up to 500 MHz with higher power can corrupt the data signal and result in return and insertion losses, open circuit inductance (OCL) and DC current imbalance that fail to meet the standards specified in clause 33 of the IEEE 802.3 standard which defines the characteristics of PD and PSE equipment.